Dr. rer. nat. Clemens Barth, Aix-Marseille University, CNRS, CINaM UMR 7325, Marseille, France

Kelvin probe force microscopy:  From the 'reactivity' of Pd nanoparticles to charge manipulation of C60 molecules

Frequency modulated noncontact atomic force microscopy (FM nc-AFM) is nowadays a well established surface science tool, which permits imaging surfaces of conducting or insulating materials with atomic resolution [1]. Recently, it has been shown that even the atoms inside single molecules can be imaged, with an unmatched resolution in comparison to scanning tunneling microscopy (STM).

However, a drawback of this technique is that it is not a priori chemical sensitive so that the chemical nature of atoms, defects and adsorbates cannot be identified as such. Furthermore, although the electrostatic contribution is included in the tip-surface interaction, the obtained images do not show in a direct way phenomena of work function (WF) and surface charges. Inspired by classical Kelvin probe [2], the Kelvin modulation technique was implemented into AFM in 1991 [3] to measure the local work function (WF) of surfaces and the distribution of surface charges and dipoles, which may assist chemical identification [1]. Since then Kelvin probe force microscopy (KPFM) is used for many applications in surface science and nanosciences thanks to its high resolution at the nanometer scale and in the Kelvin voltage [1]. However, the amount of work is still limited, in particular in comparison to pure nc-AFM and STM work.

In this contribution, I will present two examples, which shall demonstrate the enormous potential that lies in KPFM. The first application is located in the field of in heterogeneous model catalysis. It will be shown that KPFM can study reactivity related aspects of metal nanoparticles (NPs) such as carbon contamination, which will be exemplified by palladium NPs on the graphite surface [4]. In the second part of the  contribution it will be shown, that elementary charges on an insulator surface can be well detected by KPFM. Furthermore, the combination of KPFM and  electrostatic force microscopy (EFM) can be used to explicitly charge adsorbates, which will be exemplified by charge manipulation experiments at C₆₀ islands on bulk NaCl(001) [5].

1.  Barth, C.; Foster, A. S.; Henry, C. R.; Shluger, A. L. Adv. Mater. 2011, 23, 477−501.
2. Kelvin, L.; Fitzgerald, G.; Francis, W. Philos. Mag. (1798−1977) 1898, 46, 82−120 and  Zisman, W. A. Rev. Sci. Instrum. 1932, 3, 367−370.
3. Nonnenmacher, M.; O’Boyle, M. P.; Wickramasinghe, H. K. Appl. Phys. Lett. 1991, 58, 2921−2923.
4. Palacios-Lidon, E.; Henry, C. R.; Barth, C. ACS Catal. 2014, 4, 1838−1844.
5. Hoff, B.; Henry, C. R.; Barth, C., in preparation (2015)